Two step refining process in a tiltable rotary furnace



United States PatentO TWO STEP REFINING PROCESS IN A TILTABLE ROTARYFURNACE Bo Michael Sture Kalling, Domnarvet, and Folke Carl EvaldJohansson, Borlange, Sweden, assignors to Stora Kopparbergs Bergsla'gsAktiebolag, Falun, Sweden, 2 company of Sweden No Drawing. ApplicationNovember 18, 1955 Serial No. 549,220

Claims priority, application Sweden November 25, 1954 6 Claims. (01.7560) The invention relates to the refining'of pig iron and similaralloys by free oxygen and more particularly to refining molten pig ironor other alloyed or unalloyed iron containing carbon in a furnacerotatable about an axis of rotation by blowing with a gas containing atleast 40% oxygen, such as oxygen gas or air enriched in oxygen.

It is an object of the invention to provide means for carrying out therefining process at a relatively low speed of rotation. Another objectis to make feasible to use a higher degree of filling of the furnacethan is possible in ordinary rotary furnaces. Further objects will beclear to those skilled in the art from the following description.

In the U. S. patent specification No. 2,598,393 a method is describedfor refining pig iron in a rotating furnace by blowing oxygen or airenriched in oxygen to an oxygen percentage'of at least 40% into thefurnace. It is characteristic for this method that the necessary bathmovement is essentially obtained independently of the gas supply. Thisis contrary to the'conditions in other quick-refining processes wherethe bath movement is P ce tained in'the rotary furnace a relatively highspeed of rotation is suitable, however, it should be possible toincrease this speed up to 30 to 40 revolutions per minute in cases wherea particularly strong stirring is desired. During certain periods of theblowing and particularly when the carbon refining is most vivid andcontributes to connected with the gas supply and ceases entirely when Ithe latter is stopped. This is, for instance, the case in the commonacid and basic Bessemer processes as well as in the so calledLD-process, where the refining takes place with pure oxygen gas whichisblownvertically against the bath in a stationary converter.

When a stationary furnace is used and the oxygen gas is blown in fromabove the bath movement is relatiyely insignificant in the beginning ofthe process, as long as the pig iron contains a material amount ofsilicon, and also in the later stages of the process, particularly inrefining pig iron rich in phosphorous. During that phase of the processwhere the carbon refining is vivid the boiling causes, however, aviolent stirring which is important for the proceeding of the reactions.

If the process iscarried out in a rotary furnace a vivid bath movementcan be obtained during the entire process. Hereby the reactions betweenthe metal bath and the slag bath take place more rapidly and morecompletely. This makes it possible, i. e., to reduce the iron percentagein the final slag and thereby in an essential degree to increase themetallic iron yield. By the rotation it is, furthermore, possible toobtain a suificient contact between the oxygen and the bath withouthaving to blow the oxygen onto or into the bath at a high superpressure, which is necessary with a stationary furnace.

The oxygen twyer need neither be directed against the bath. ,In thismanner the oxygen gas will to a certain extent be mixed with the furnacegases before it reaches the bath and the contact with the latter will bespread over a relatively great surface. Hereby the consider- I ablegasification of, particularly, iron and manganese is the stirring it hasappeared, however, that the speed can without disadvantage be loweredconsiderably.

The possibility of running the furnace with a lower velocity at thoseoccasions during the process when this is possible without disadvantagefrom a metallurgical point of view should be made use of as, thereby,amongst other things a considerably increased life of the lining of theconverter can be attained. It would seem, namely, that at least undercertaincircumstances a shortened life of the lining is to be reckonedwith at increased speed of rotation.

The present invention involves a further development of the art referredto and it comprises an improved method as well as a furnace for carryingout such method. According to the new method the blowing with the oxygencontaining gas is performed during at least one portion of the processwith the furnace rotating with its axis of rotation horizontal orinclined, suitably not more than 45,-the oxygen containing gas beingintroduced through a central opening of the one end wall of the furnace,and during at least another portion of the process with its axis ofrotation in vertical or approximately vertical position, the oxygencontaining gas being supplied to the bath from above. The method canwith advantage be carried out in a furnace comprising a refractory drumhaving two opposite ends, one being closed and the other having anopening, said drum being rotatable about an axis'of rotation passingthrough said closed end and said opening, means for suspending said drumswingably so that said axis of rotation can be altered from asubstantially horizontal or inclined position to a subst-antiallyvertical position, means being provided for rotating the furnace aboutthe axis of rotation at least in said substantially horizontal orinclined position. The furnace may be made swingable by supporting ittogether with the driving machinery ontrunnions. When the drum is raisedin a vertical position the rotation has onlya relatively low stirringeffect. Therefore the blowing can take place with equally good resultwith a stationary furnace, that is to say according to the principle ofthe LD-process.

Thomas pig iron or other pig iron with a phosphorous percentage of, forinstance, 0.5 or more can be blown in a furnace equipment of this kind,for instance accord ing to the following example.

After the furnace has been charged with 30 tons (metric) of moltenordinary Thomas pig iron containing 0.5% Si, 2.0% .P and 1.0% Mn, and3000 kgms. of lime and 2500 kgms. of ore have been added as coolingmedium, oxygen gas is blown into the furnace for approximately 15minutes with the latter in an approximately horizontal position with thefurnace axis at about 20 to the horizontal and with a relatively highspeed of rotation of about 30 revolutions per minute in order that thestrong smoke formation at the oxidation of silicon phosphorus andmanganese shall be counteracted. After said blowing period when theboiling sets in seriously and gives rise to a sufficient stirring, andthe rotation is interrupted and the furnace is raised to a substantiallyvertical position. In connection therewith a slag is tapped, and afurther quantity of 1000. kgms. of limestone and 1500 kgms. of ore ischarged to the bath in the furnace. The blowing is then continued byblowing oxygen, through a vertical oxygen twyer, against the bath underconditions to displace slag and bring the oxygen stream into directcontact with the surface of the underlying metal. The furnace is notrotated during this phase which lasts about 10 minutes and brings thecarbon percentage down below 0.3%. At the final phases of the process,which takes about 3 minutes, the blowing is again carried out with thefurnace rotating in a substantially horizontal position in order thatthe speed of reaction between the slag and the steel bath shall beincreased and thereby the iron losses in the slag be reduced. After thisphase the steel is tapped in a ladle as in conventional practice.

If desired and for the purpose of cooling the bath during the processore as well as scrap iron can be added continuously or at suitableintervals. If scrap ir'o-n of a coarse kind is to be added to the moltenbath it should preferably be supplied immediately before raising thefurnace into a vertical position. Finer scrap iron can be added withoutdisadvantage also before or during the rotation. I

Pig iron of other types may be refined by similar meth ods. As a rule,it is suitable to limit the timeof blowing in the rotary furnace as faras possible without unfavourably influencing the progress of the processthereby, and to carry out the rest of the blowing, usually under anintermediate period when the carbon refining is vivid, with the furnacestationary or only slowly rotating in vertical position. In such mannerthe life time of the furnace lining can be considerably increased andthe advantages of the rotation effect simultaneously be fully utilized.

During the boiling period the gas development always causes an increaseof the volume of the charge which can be considerable, particularly ifthe slag simultaneously formed in the furnace gets into foaming, whichis often the case. Owing to the fact that the permissible degree offilling of the furnace is greater in the vertical position than in thehorizontal it is consequently possible to use a greater charge in thefurnace if the blowing is carried out with the furnace in the more orless vertical position, at least during the most vivid boiling. Anincrease of the blowing speed or intensity is also obtainable with thismode of operation.

It is an advantage with the present method that it is particularly wellsuited for refining pig iron of high silicon percentages. If thepercentages of silicon exceeds l to 1.5% it has appeared to be difficulttostart the reaction at the beginning of the blowing if the process iscarried out in a stationary furnace. By rotation of the furnace so thata vivid bath movement is produced the reaction starts immediately andwhen the percentages of silicon has been reduced the blowing may becontinued with the furnace in vertical position when found suitable fromother points of view.

What is claimed is:

1. In refining molten ferrous metal containing carbon in a tippablefurnace rotatable about an axis of rotation by blowing the molten metalwith a gas containing at least 40% free oxygen the improved method whichcomprises performing said blowing during a stage of the process duringwhich the furnace rotates with its axis of rotation inclined not morethan 45 from the horizontal the oxygen containing gas being introducedthrough a central opening in an end wall of the furnace and duringanother stage of the process during which the axis of rotation of thefurnace is approximately vertical, the oxygen containing gas beingsupplied to the molten metal from above. 7

2. The method defined in claim 1 in which the first stage of the blowingtakes place with thefurnace rotating with the axis of rotation in aninclined position, and in which silicon and manganese present in themetal are oxidized while iron losses into the slag are reduced.

3. The method defined in claim 1, in which the last stage of the blowingis carried out with the furnace rotating with the axis of rotation inthe inclined position for reducing the iron losses in the slag.

4. The method defined in claim 1, in which the blowing is carried outduring at least a portion of the so called boiling period with the axisof rotation of the furnace in the substantially vertical position.

5. The method defined in claim 1, characterized in that the metal to berefined has a silicon content higher than 1%.

6. The method defined in claim 1 characterized in that the metal to berefined contains more than 0.5% P.

References Cited in the file of this patent UNITED STATES PATENTS 76,581Allen Apr. 14, 1868 2,258,850 Eulenstein et a1. Oct. 14, 1941 2,598,393Kalling et a1. May 27, 1952 FOREIGN PATENTS 677,349 Great Britain Aug.13, 1952

1. IN REFINING MOLTEN FERROUS METAL CONTAINING CARBON IN A TIPPABLEFURNACE ROTATABLE ABOUT AN AXIS OF ROTATION BY BLOWING THE MOLTEN METALWITH A GAS CONTAINING AT LEAST 40% FREE OXYGEN THE IMPROVED METHOD WHICHCOMPRISES PERFORMING SAID BLOWING DURING A STAGE OF THE PROCESS DURINGWHICH THE FURNACE ROTATES WITH ITS AXIS OF ROTATION INCLINED NOT MORETHAN 45* FROM THE HORIZONTAL THE OXYGEN CONTAINING GAS BEING INTRODUCEDTHROUGH A CENTRAL OPENING IN AN END WALL OF THE FURNACE AND DURINGANOTHER STAGE OF THE PROCESS DURING WHICH THE AXIS OF ROTATION OF THEFURNACE IS APPROXIMATELY VERTICAL, THE OXYGEN CONTAINING GAS BEINGSUPPLIED TO THE MOLTEN METAL FROM ABOVE.